LONDON (Reuters) - Six years after the September 11 attacks in the United States, the "war on terror" is failing and instead fuelling an increase in support for extremist Islamist movements, a British think-tank said on Monday.

A report by the Oxford Research Group (ORG) said a "fundamental re-think is required" if the global terrorist network is to be rendered ineffective.

"If the al Qaeda movement is to be countered, then the roots of its support must be understood and systematically undercut," said Paul Rogers, the report's author and professor of global peace studies at Bradford University in northern England.

"Combined with conventional policing and security measures, al Qaeda can be contained and minimized but this will require a change in policy at every level."

He described the U.S.-led invasion of Iraq as a "disastrous mistake" which had helped establish a "most valued jihadist combat training zone" for al Qaeda supporters.

The report -- Alternatives to the War on Terror -- recommended the immediate withdrawal of all foreign troops from Iraq coupled with intensive diplomatic engagement in the region, including with Iran and Syria.

In Afghanistan, Rogers also called for an immediate scaling down of military activities, an injection of more civil aid and negotiations with militia groups aimed at bringing them into the political process.

If such measures were adopted it would still take "at least 10 years to make up for the mistakes made since 9/11."

"Failure to make the necessary changes could result in the war on terror lasting decades," the report added.

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Rogers also warned of a drift toward conflict with Iran.

"Going to war with Iran", he said, "will make matters far worse, playing directly into the hands of extreme elements and adding greatly to the violence across the region. Whatever the problems with Iran, war should be avoided at all costs."